3 Popular Diets That Actually Work (and 3 That Don't)

In the world of health and wellness, there's nothing more confusing than dieting. Sure, we're told time and time again the only thing that really works is diet and exercise (and patience). Yet we're still willing to listen and attempt every new fad in hopes that it's the one that changes everything. (Usually, it's not.)

So we decided to declutter our minds and find out once and for all which popular diets really work. Together with Compare the Market, we put six diets to the test and had a nutritionist rate each one on a five-star scale based on effectiveness, ease, and sustainability. Keep reading to find out which trendy diets made the cut (and the ones that didn't).

What is it? Most popular in the U.S., Canada, and Australia, this diet is inspired by the dietary habits of the Mediterranean. It promotes the consumption of fresh, non-processed foods including vegetables, olive oil, fish, and chicken.

What can I have? You can eat plenty of fresh fruits and vegetables, white meat (fish and chicken), olive oil, nuts, whole grains, legumes, herbs and spices, cheeses, and red wine.

What can't I have? You have to stay away from processed foods, salt, red meat, and saturated fat.

The pros: Of course, the most palpable pro is that consumption of red wine is included in the program, along with the fact that the majority of the allowed foods are associated with preventing heart disease, diabetes, high blood pressure, and obesity.

The cons: While living off of red wine, fresh vegetables, cheese, and fish sounds enticing, it can also be quite expensive to maintain. Plus, many unsaturated fats are still present in the meal plan (including cheese and oils).

What the nutritionists say: Shereen Lehman, MS, an evidence-based nutrition specialist and the co-author of Superfoods for Dummies, speaks to the ease of the diet: "The Mediterranean diet includes a lot of delicious food and has good variety. Of course," she admits, "if you need to lose weight, you'll still need to count calories." Lehman continues, "It's healthy, nutritious, and the health benefits are there. With evidence of effectiveness for cardiovascular and overall health, this diet focuses on what foods are good for you.

It's high in omega-3 fats, monounsaturated fats (olive oil), fiber, calcium, and it is pretty much everything a healthy diet should be."

What is it? The 5:2 diet is most popular in the United Kingdom, Australia, and Sweden, and consists of two days following a restricted calorie fast and five days of normal eating.

What can I have? For five days out of the week, you can continue your normal diet of about 2000 to 2500 calories.

What can't I have? While there aren't any preset food restrictions, you're instructed to fast for two days out of the week. On those days, you have to adhere to a 500-calorie-per-day regimen (600 calories for men).

The cons: While, technically, fasting for two days can be just as effective as continuous calorie restriction, it can also severely impact your energy levels and lead to nutrient deficiencies. If you plan to work out, you'll have to do so on days when you're not fasting.

What the nutritionists say: Meryl Pritchard, a holistic nutritionist and founder of organic meal delivery service Kore Kitchen, weighs in: "If done right, fasting can be very beneficial to your health," Pritchard says. "It allows your body to repair and rejuvenate. This is what your body does when you're sleeping. If you're constantly eating, your body doesn't have the opportunity to do this." But, she explains, "Most people don't even know what a calorie is, let alone how to measure it," she says.

"What we tell our clients is that it's really not about the quantity of calories—you should focus more on the quality."

What is it? A juice cleanse diet consists of drinking freshly pressed or blended fruits and vegetables, and avoiding the consumption of solid foods for a specific amount of time, usually anywhere between a couple of days and a number of weeks. It's most prominent in the U.S., Canada, and Singapore.

What can I have? You can have juiced fruits and vegetables as well as spices like ginger and cayenne pepper.

What can't I have? You have to stay away from all solid foods and alcohol.

The pros: This diet encourages fruit and vegetable consumption while reducing your fat intake. As such, it results in fast weight loss.

The cons: Juicing separates out the fiber needed to aid in your body's digestion, and, because it limits your diet so much, you may gain weight back quickly after returning to your normal diet. Also, juices can be quite high in sugar.

What the nutritionists say: "These trendy cleanses do not fulfill a lifetime of happiness and health but rather a couple days," says Caroline J. Cederquist, MD, creator of BistroMD, a doctor-designed and chef-prepared meal delivery service. "Adding juicing into your diet can be beneficial to receiving vital nutrients; however, juicing should not be the only thing in your diet." In fact, "you may hear a lot about the value of juice cleanses, but the fact is that there is no real scientific evidence to back it up," says certified nutrition coach Candice Seti, Psy.D., of The Weight Loss Therapist.

What is it? The Paleo diet claims that modern foods have evolved much faster than our bodies, causing problems and illnesses. The regimen promotes eating foods that early Paleolithic humans might have eaten and is most popular in Australia, New Zealand, and the U.S.

What can I have? On the Paleo diet, you can eat grass-fed meats, seafood, fresh fruits and vegetables, eggs, nuts, seeds, and healthy oils including coconut, avocado, olive, and walnut.

The pros: Your diet will be cleaner and certainly contain fewer processed foods, and, because it's high in protein, you'll feel full longer. The plant nutrients from fruits, vegetables, and oils can have anti-inflammatory benefits, and you won't have to count calories.

The cons: While there are a lot of nutritious pros, the diet does lack certain nutrients including calcium and vitamin D. Plus, it can be particularly difficult for vegetarians, as beans aren't allowed.

What the nutritionists say: Lehman says, "Strict rules can make dieting easy at first, but it gets harder in the long run. Because Paleo is popular, it can be made easier with available Paleo-approved snacks and recipes." But, unfortunately, the diet isn't especially balanced. According to Lehman, "The Paleo diet excludes a lot of healthy foods. If you really want to follow it, choose fish for healthy fats and eat a lot of fruits and vegetables, especially dark, leafy greens for calcium."

What is it? An alkaline diet involves eating alkaline foods in order to keep your body's pH levels between 7.35 and 7.45 (14.0 is pure alkaline, 7.0 is neutral, and 3.0 is acidic), avoiding foods that become acidic once broken down. The diet claims to improve memory and boost energy levels while preventing headaches and bloating.

What can I have? On the diet, you can have fresh fruits (including citrus), vegetables, nuts, and legumes.

What can't I have? You can't, however, have any wheat, pasta, meat, fish, shellfish, dairy, tea and coffee, sugar, or alcohol.

The pros: You'll definitely increase your vegetable and fruit consumption, which can contribute to a reduction in muscle pain. There will also be a drastic reduction in sugar and fat intake.

The cons: This plan removes whole food groups, and sometimes, knowing what you can and can't eat can be confusing.

What the nutritionists say: "I like that this diet is high in fruits and vegetables," says Lehman. But "restrictions are difficult to follow, and this one is no exception: no gluten, little to no meat, no processed foods, and no alcohol or caffeine." Basically, increasing your fruit and vegetable intake is highly beneficial for your health, but restrictiveness may reduce the effectiveness of this diet. However, if you already adhere to a plant-based diet, it might be an easier transition.

What is it? The Blood Type Diet recommends different dietary needs depending on your blood type, as foods react chemically to different blood types.

What can I have? The food guidelines are broken down based on your blood type: Type O is meant to eat high protein, lean meat, poultry, fish, and vegetables; type A has fruits, vegetables, beans, legumes, and whole grains; type B is restricted to green vegetables, eggs, certain meats, and low-fat dairy; type AB should eat tofu, seafood, dairy, and green vegetables.

What can't I have? Type O has to stay away from grains, beans, and dairy while type A can't have meats and heavy proteins. Type B is restricted from corn, wheat, buckwheat, lentils, tomatoes, peanuts, sesame seeds, and chicken, while Type AB is cannot have alcohol, caffeine, and smoked or cured meats.

The cons: While the Blood Type Diet allows for a lot of healthy foods, it is also limiting and cuts out entire food groups.

What the nutritionists say: "You'll cut out a lot of the junk food you're currently eating, which can lead to weight loss," says Lehman, but "remember, you still have to make healthy choices from the foods you are allowed to eat." She continues, "The Blood Type Diet is an interesting hypothesis, but there's no evidence to show that it works. I don't recommend it."